High quality, intensive early intervention is a powerful treatment for ASD, improving IQ and language markedly in randomized controlled clinical trials (RCTs), though little long term follow-up data exists. Few core characteristics that affect child change have been tested. Two potential core characteristics that invoke considerable debate among parents, professionals, and administrators are the delivery style of intervention: play-based versus discrete trial teaching, and the intensity (dosage) of intervention. This ACE treatment network will conduct two RCTs to answer two main questions: study 1: what are the effects of intensity and delivery style on developmental progress of toddlers with ASD, and study 2: do toddlers from a previous RCT of a specific intervention approach, the Early Start Denver Model (ESDM), versus community treatment maintain and generalize treatment effects over 3 years? For study 1, a sample of 108 young children with ASD, ages 15-30 months, will be enrolled in one of three national sites (Univ Cal Davis, Vanderbilt, Univ. Washington) and randomized into one of four cells varying on two dimensions: dosage - 15 or 25 hours per week of 1:1 treatment; and direct or play- based teaching. All other aspects of intervention will be hel constant: use of the principles of applied behavior analysis, content of the teaching, location, staff, parent curriculum, and materials. Developmental progress will be measured monthly allowing for growth curve analysis to examine fine-grained differences in groups as well as interactions among major child and family initial variables and these two experimental variables. For study 2, 79 participants from a previous ACE Network project enrolled between 15 and 24 months of age will be followed up at 6-7 years of age. All children were randomized into two years of intensive intervention of either ESDM or community intervention and assessed with a comprehensive diagnostic and behavioral assessment battery at four time points during the original study. Stability of treatment effects and group differences will be examined. Specific aims are: 1. to examine the effect of intensity of treatment, or dosage, on child progress/ outcomes; 2. to examine the effect of intervention delivery style: play-based or DTT, on child progress/ outcomes; 3. to examine effects of quality and quantity of parent-provided learning opportunities at home on child progress/ outcomes; 4. to test the moderating effects of initial pre-linguistic vocal patterns, autism severity, and developmental rates on child outcomes; 5. to examine maintenance of ESDM treatment child and family effects three years post intervention; 6. to support the scientific development of young investigators with specific interest and expertise in autism science.